Seasonal Shifts and Monsoons
| Since the planetary winds
are caused by heat from the sun, they shift northward and
southward as the sun changes position with the seasons.
The wind shift lags behind the sun's position. When heat
from the sun is increasing, it is absorbed by the Earth
for a time before the air temperature rises. When the
heat from the sun is decreasing, the stored heat is given
out and air temperature is maintained. The oceans maintain their temperatures and the winds over them do not have large seasonal changes. Land surfaces cannot absorb and give out enough heat to do this. Over the continents, the air shows marked seasonal changes of temperature. This occurs about two or three months after the amount of heat from the sun decreases or increases. Monsoon winds can be explained by the great seasonal temperature changes over large landmasses. In India and Southeast Asia, for example, the winds of summer blow mostly from the Indian Ocean and the South China Sea into the heated continent. These summer monsoon winds usually bring heavy rainfall. The winds of winter, however, blow mostly outward from the cold, continent. These winter monsoons are usually dry. The seasonal reversal of the monsoon is also due to the shifting of the upper-level west-to-east winds known as the jet stream. The northward shift of the jet stream in summer allows moist maritime air to move into Asia. The southward shift of the jet stream in winter blocks it. |